Along with the trend toward smaller portable electronic devices in recent years, lithium ion batteries and nickel-metal hydride storage batteries are becoming widely used as a power source for such devices. Because lithium ion batteries and nickel-metal hydride storage batteries are highly resistant to vibration and impact, attention has been paid to these batteries as a power source for devices that require a large current such as a cordless electric tool, a power assisted bicycle and a hybrid automobile.
Development has also been underway on the shape of batteries to cope with the devices in which the batteries are used, such as cylindrical batteries and flat batteries. In either shape, reductions in size and weight, and increased power output are required.
FIG. 31 shows a general structure of a secondary battery as typified by a lithium ion battery. A battery 100 shown here includes an electrode group 108 in which a positive electrode plate 102 that employs a lithium-containing composite oxide as an electrode active material and a negative electrode plate 104 that employs a material capable of retaining lithium as an electrode active material are spirally wound with a separator 106 interposed therebetween. The electrode group 108 is housed in a bottomed cylindrical battery case 110 together with insulating plates 112 that are disposed on the top and bottom thereof.
The negative electrode plate 104 is connected to the bottom portion of the battery case 110 by a negative electrode lead 114 taken out from a lower portion of the electrode group 108. The positive electrode plate 102 is connected to a sealing plate 118 by a positive electrode lead 116 taken out from an upper portion of the electrode group 108. In addition, a prescribed amount of non-aqueous electrolyte (not shown) is injected into the battery case 110. The opening portion of the battery case 110 is sealed by folding the opening edge of the battery case 110 inward so as to crimp onto the peripheral portion of the sealing plate 118 over a sealing gasket 120 attached to the periphery of the sealing plate 118.
As described above, in a conventional secondary battery, a belt-shaped positive electrode lead and a belt-shaped negative electrode lead are respectively attached to the current collectors of a positive electrode plate and a negative electrode plate, and the positive electrode plate and the negative electrode plate are respectively connected to a sealing plate and a battery case that serve as external terminals via the leads. However, such a connecting method is problematic in that in a high power output secondary battery that requires a large current, in particular, an IR loss (voltage loss) in the positive electrode lead and the negative electrode lead becomes large, resulting in the degradation of discharge characteristics.
Patent Document 1 has proposed a cylindrical battery for solving the above problem. This battery includes a current collector portion that is in multiple-point contact or multiple-point connection with a spirally wound electrode group (not shown) that is housed in a battery case (see FIG. 1 of Patent Document 1). The current collector portion and a sealing lid are connected by a belt-shaped lead portion and also by a metal spring. With this configuration, the IR loss is reduced, as a result of which the high rate discharge characteristics are improved.
Patent Document 2 has proposed the following non-aqueous electrolyte secondary battery. In this battery, a positive electrode current collector exposed portion protrudes on the upper side of the electrode group (see FIG. 1 of Patent Document 2). Also, a negative electrode current collector exposed portion protrudes on the lower side of the electrode group. On the top and bottom of the electrode group, conductive elastic bodies are disposed so as to make contact with the upper and lower exposed portions, respectively. On the upper side further away from the upper elastic member, a spring for urging the power generating element downward is disposed. With this configuration, the IR loss is reduced, as a result of which the high rate discharge characteristics are improved.    Patent Document 1: Japanese Laid-Open Patent Publication No. H06-36756    Patent Document 2: Japanese Laid-Open Patent Publication No. 2000-311714